1. Field of the Invention
The present invention relates to a common voltage regulating circuit of a liquid crystal display device, and more particularly to a common voltage regulating circuit of a liquid crystal display device capable of adjusting a common voltage by software.
2. Description of the Prior Art
Generally, a TFT-LCD is a device for displaying an image by adjusting light transmissivity after changing an orientation of a liquid crystal through charge/discharge of capacitors formed between pixel electrodes and common electrodes. A signal voltage is applied to the pixel electrode through a data line and TUFT performing a switching, and a common voltage is applied to the common electrode. Herein, in order to minimize flicker, the common voltage is finely adjusted by means of a common voltage regulating circuit to a predetermined value.
FIG. 1 is a schematic diagram showing a common voltage regulating circuit of a liquid crystal display device according to a prior art. As shown in FIG. 1, the common voltage regulating circuit includes a voltage distribution section 10 and a buffer amplifier 20. The voltage distribution section 10 includes three resistors, that is, a first resistor R1, a second resistor R2 and a variable resistor VR1, which are coupled in series between a power supply terminal and a ground, and distributes supply voltage. The buffer amplifier 20 includes a capacitor C1 coupled between an output terminal and a ground, receives a distribution voltage, which is a reference voltage, adjusted by the variable resistor VR1 through a non-inverting input terminal (+), and feedbacks an output signal VCOM to a inverting input terminal (−). Further, the buffer amplifier 20 outputs a common voltage signal VCOM after buffering the adjusted distribution voltage.
FIG. 2 is a front view of a liquid crystal display panel employing the circuit of FIG. 1. A reference numeral 100 represents a width of a bezel of front surface of the liquid crystal display panel and a reference numeral 102 represents a groove for adjustment of the resistance of the variable resistor.
FIG. 3 is a rear view of a liquid crystal display panel employing the circuit of FIG. 1. As shown in FIG. 3, the liquid crystal display panel includes a source-drive IC 104 for driving a data line of the liquid crystal display panel, a gate-drive IC 106 for driving a gate line of the liquid crystal display panel, a source-printed circuit board 108 for supplying a power source and a driving signal to the source-drive IC 104, a gate-printed circuit board 110 for supplying a power source and a driving signal to the gate-drive IC 106, a first cable 112 for connecting the source-printed circuit board 108 with the gate-printed circuit board 110, and an integrated board 114 with which a liquid crystal driving circuit, which drives the liquid crystal display panel, and an interface circuit, which converts an input image signal such as a LVDS, a TTL, or a TMDS into a digital type and adjusts the resolution, have been formed integrally. Further, the liquid crystal display panel includes a second cable 116 for connecting the source-printed circuit board 108 with the integrated board 114, an inverter 118 for driving a backlight of a liquid crystal display, a connector 120 for sending the image signal to the integrated board 114, a third cable 122 for connecting the integrated board 114 with the inverter 118 and a variable resistor 124 utilized for a fine adjustment of the common voltage.
FIG. 4 is a rear view of another embodiment of a liquid crystal display panel employing the circuit of FIG. 1 and is a schematic view showing rear surface of the liquid crystal display panel in which the interface circuit and the inverter are omitted. In the following description and drawings, the same reference numerals are used to designate the same elements as those shown in FIG. 3.
As shown in FIG. 2 and FIG. 3, the conventional common voltage regulating circuit is installed on the gate-printed circuit board 110. The integrated board 114 includes a block of generating the supply voltage AVDD and supplies the supply voltage AVDD to the source-printed circuit board 108 and the gate-printed circuit board 110 through the second cable 116. Herein, the supply voltage AVDD is sufficiently larger than the level of the common voltage VCOM outputted from the common voltage regulating circuit.
Referring to FIG. 2 and FIG. 3, the operation of the common voltage regulating circuit is briefly described. First, when the supply voltage AVDD generated in the integrated board 114 is supplied to the common voltage regulating circuit, the voltage distribution section 10 distributes the supply voltage AVDD by means of the first resistor R1, the second resistor R2 and the variable resistor VR1 on the basis of a value set by the variable resistor VR1, and then outputs the distributed voltage, which is a reference voltage, to the buffer amplifier 20. Then, the buffer amplifier 20 amplifies the reference voltage by a unity gain and then outputs a stable common voltage signal VCOM.
In the conventional common voltage regulating circuit, parts, such as a cheap transistor, have been used as means for outputting the stable common voltage signal, or the output of the variable resistor has been directly used as the common voltage signal.
When the liquid crystal display device is manufactured by means of the conventional common voltage regulating circuit as described above, as shown in FIG. 2 and FIG. 3, a groove for adjustment of a resistance of the variable resistor must be formed usually on the front surface of the panel, or sometimes on the rear surface of the panel. Therefore, when the bezel inevitably has a narrow width in designing the liquid crystal display device, there are some restrictions in forming the groove. Further, when a liquid crystal display device has no gate-printed circuit board, the position of the variable resistor must be shifted to the source-printed circuit board, thereby causing some difficulties in the structural design of the liquid crystal display device.
Further, when the liquid crystal display device is manufactured to have the conventional common voltage regulating circuit, there are some difficulties in fine adjustment in accordance with the accuracy of the variable resistor, and the variable resistor may be broken owing to a structural defect. Further, as the variable resistor is used, the manufacturing cost increases.
Further, when a complete display, such as a monitor, has been manufactured by means of a liquid crystal display device having the conventional common voltage regulating circuit after completing the adjustment of the common voltage, it is impossible to readjust the common voltage unless the display device is disassembled.